This Yast is about the part which "Ab", Water, takes in the cycle of Cosmic Power (Star Energy): down from the stars who irradiate it, from around, into the clouds of Water in the Atmosphere. Whichever Constellation is present at the time, will determine the nature and composition of this radiating Energy. From the Clouds it comes down in the Water of rain, to penetrate into the inorganic matter of Earth : rock, sand and mud. The inorganic Rock will get pulverized into sand through the ages, to form the earth in which organic, living Plants will grow. The Plants, in turn, will absorb the Cosmic energy with the Water they ingest. This way, " Love is like Water", indeed, as one of the Books of Wisdom says ( Ardvi Sura Anahita works as one of the three powers of Sraosha here). The water circulates like blood through a body and its organs, to nurture it with Cosmic energy where needed. And when the Plants consequently exchange carbon-dioxide for oxygen with the Air, they get a chance to release the Cosmic Energy, for "Living Beings", animal and human, to be inhaled. They in turn must spiritualize this energy, in the Sanskrit tradition known as "Prana", and return it back by means of good thoughts and feelings : to strengthen the Cosmic Unity, maintaining its Harmony through prayers and songs, uttered "on Breath". Now we understand, why next to water, plants are mentioned in this Yast !

I [Zarathustra spoke:]

May there be invocation, with sacrifice and prayer, to the Water that was made by Mazda, to the Holy Water-Springs : to all Waters, that were made by Mazda, and to all Plants, made by Mazda !

II And Ahura Mazda  spoke to Zarathustra, saying :

“ O, Zarathustra ! do offer up a sacrifice to this Spring of mine :  Ardvi Sura Anahita, who is so wide expanding and Health-giving, who hates the Devas and  obeys the Laws of Ahura,  who is worthy of invocation in the material world.

It is She who is the Life-increasing and Holy One, the Wealth-increasing and Holy One, and the Country-increasing and Holy One : who makes the seed of all males pure, who makes the womb of all females pure for bringing forth, who makes it possible for all females to bring forth in safety, who puts milk into the breasts of all females in the right measure and in the right quality. 

III That large River, known afar, that is as large as the whole of the Waters that run along the Earth, that run powerfully from the Heights : from this River alone do flow all the waters that spread all over the seven Kingdoms !

IV This River of mine alone does go on bringing Waters, both in Summer and in Winter ; V this River of Mine does purify the seed in males, the womb in females, the milk in females’ breasts : this river is, VI She, who sends down by Day and by Night a flow of motherly Waters as large as the whole of the Waters that does run along the Earth, and does run powerfully !

VII To Her shall the followers of  ASHA  offer up a sacrifice : with wisdom of the tongue, with Holy spells, with Words : with Deeds and with rightly-spoken Words !”